This invention relates to sensors for measuring or monitoring the thickness of thin sheet material traveling along a fixed path adjacent the sensor. More particularly, this invention relates to sensors of the aforedescribed type which measure or monitor very thin articles such as thin sheets of paper or the like and can detect variations in thickness such as a double sheet fed along the path. Still more particularly, this invention relates to sensors of the aforementioned type which employ optoelectronic triangulation measuring means, and which utilize a pivoted lever to bias the articles against a stationary platen to establish a reference position for the sensor.
Sensors which monitor the thickness of a thin flexible web of material or of individual thin sheets, such as paper traveling along a known path, can also be used to detect an overlapped or multiple thickness condition of plural sheets of paper traveling along that path. Electromechanical and through-beam photoelectric sensors are commonly used for thickness monitoring but are generally unsuitable for detecting thin sheets of paper or thick card stock inasmuch as they are difficult to adjust for such accuracy. Optoelectronic triangulation sensors provide the desired accuracy to detect the thickness of thin sheets of paper and to distinguish a single sheet from two or more sheets. Such sensors optically measure the distance from a reference surface supporting the paper to the top surface of the paper. However, optoelectronic triangulation sensors are non-contact sensors and do not determine if the paper is in contact with the reference surface, but measure only the distance to the top surface of the reference platen and to the top surface of the paper. If the paper is above the reference platen, an inaccuracy results.
Inaccuracies can also result when sensing printed papers wherein the light emitted from the sensor is focused on the paper in areas having black print or images. When the focused light spot is in a white area, the center of the spot is its orthogonal center. As the spot is focused at least partially on a black printed area, the center shifts to an integrated center of the white portion of the spot. This degree of inaccuracy, although very small, is not acceptable in applications sensing papers having a thickness approximately 0.002 inches.
Still other disadvantages of available sensors of the aforementioned type are that they are slow, provide digital outputs and are expensive. Most such sensors require a pulse modulated light source which adds complexity and cost to the control circuit. Those which provide an analog output signal employ diode arrays which are sequentially scanned to determine the output signal.
This invention overcomes the aforedescribed disadvantages.